Length and Diameter Control of Ultrathin Nanowires of Substoichiometric Tungsten Oxide with Insights into the Growth Mechanism

Abstract

Tungsten oxide ultrathin nanowires have potential applications in electrochromic devices, dye sensitized solar cells, gas sensors, and as photocatalysts. Herein, we report a synthesis for solution phase ultrathin nanowires with independent control over the length and diameter. W(CO) 6 is used as the tungsten source, and the solvents octadecanol and octadecene are used. Morphological control is obtained by varying the ratios between these three components as well as reaction conditions such as time and temperature. Such precise synthetic tuning will enable future investigations on the role of the aspect ratio and diameter for the above-mentioned applications. We can infer from experimental data a plausible nucleation pathway that involves the formation of tungsten alkoxide clusters. Raman spectroscopy, X-ray photoelectron spectroscopy, electron microscopy, and X-ray and electron diffraction are used to characterize the nanowires, and the results indicate the phase to be a crystallographic sheer structure, such as W 20 O 58 . ■ INTRODUCTIONTungsten oxide is perhaps most well-known for its electrochromic (EC) properties. Chromism in tungsten oxide was first demonstrated by Berzelius in 1815 by flowing hot H 2 gas over WO 3 to effect a color change. 1 However, the ability to apply an electric field to induce either transparency or visible light absorbance was demonstrated by S. K. Deb in 1969. 2 This technology is now the foundation for so-called smart windows. In spite of the large number of publications on tungsten oxide electrochromism, research is still being done to improve upon relevant properties such as coloration efficiency, stability, and coloration/bleaching time. Traditionally, the highest rates and efficiencies came from amorphous tungsten oxide films grown from vacuum deposition techniques. The drawbacks to amorphous films include their limited chemical stability and the fact that repeated ion insertion/removal degrades the films. 3 However, nanocrystalline films have recently been shown to demonstrate similar coloration efficiency and equivalent coloration times while showing greater stability. 3,4 These improvements for nanoparticle films were attributed to their large surface areas and low packing density. This rationale led to the use of vertically oriented nanowire (NW) arrays of tungsten oxide as an EC device layer and resulted in fast coloration/ bleaching times and enhanced contrast ratio between colored and bleached states. 5,6 In addition to the crystallinity and morphology, stoichiometry of the crystalline phase is an important parameter for tungsten oxide based EC device performance. It has been previously demonstrated that coloration efficiency of the substoichiometric W 20 O 58 is higher than that for WO 3 films. 7 W 18 O 49 has also been shown to exhibit good EC properties. 8 Apart from EC applications, tungsten oxide has also been shown to be useful in a number of other applications including photocatalysis, gas sensing, dye sensitized solar cells, optical recording devices, and high T ...

Highly Flexible Sub‐1 nm Tungsten Oxide Nanobelts as Efficient Desulfurization Catalysts

“…[ 13 ] Mokari et al succeeded in obtaining freestanding ultrathin W 20 O 58 nanowires with controllable length and diameter, utilizing 1-octadecanol as the shape controlling agent and solvent and W(CO) 6 as the tungsten source. [ 14 ] Li et al controlled removal of surfactant from the presynthesized mesolamellar precursor and got W 18 O 49 nanowires with high aspect ratio. [ 15 ] Works mentioned above are nanorod or nanowire cases.…”

Nanowires and Nanostructures that Grow like Polymer Molecules

“…[5 ] Copyright 2012 American Chemical Society. g-l) Growth kinetic of tungsten oxide nanowires: [ 10 ] g-i) TEM micrographs of aliquots obtained after 6 min (g), 10 min (h), and 14 min (i) scale bars are 50 nm; l) graph of the measured (black empty squares) average contour length of the nanowires as a function of reaction time, and the fi t of the data (red line) obtained by using the model described by Cademartiri et al [ 5 ] Adapted with permission. [ 10] Copyright 2013, American Chemical Society.…”

“…Anisotropic monomers are often invoked to justify the purely one-dimensional growth of nanowires. [ 10 ] …”

“…[ 9 ] Moshofsky et al recently synthesized ultrathin nanowires of substoichiometric tungsten oxide by the solution phase reaction of tungsten hexacarbonyl in a mixture of 1-octadecanol and octadecene (see Figure 10 g-i). [ 10 ] The synthesis afforded independent control of the length and diameter of the nanowires, by varying the concentration of ligands, the solvent, the precursors and the temperature. The authors determined the growth rate of their nanowires by measuring the length of nanowires grown for different times from TEM images.…”

From 1D to 3D – macroscopic nanowire aerogel monoliths

“…In addition, recent advances in the synthesis have made it possible to produce ultrathin (<10 nm) tungsten oxide nanowires with high aspect ratios. [19][20][21][22] It was shown that ultrathin tungsten oxide nanowires could be assembled into 2D thin films with tunable thickness over a very large area, exhibiting outstanding performance in gas sensing, in electrochromic devices and in photodetectors. [22][23][24] However, the assembly of these 1D nanostructures into 3D architectures remains an open task.…”